1. Field of the Invention
This invention relates to rotary head devices, and more particularly to rotary head devices for guiding a tape-shaped medium to a peripheral surface to record or reproduce signals on or from the tape-shaped medium.
2. Description of the Related Art
FIG. 1 to FIG. 5 are figures for explaining about the conventional rotary head device.
In FIG. 1, a cylinder C is constructed with an upper drum 1 and a lower drum 2, between which exists a slit 3. A magnetic tape 4 (recording medium) is trained round the cylinder C. A magnetic head 5 is fixed to the upper drum 1 and its sliding surface 6 (to be described more fully later) with the magnetic tape 4 is made to protrude outwardly of the peripheral surface of the upper drum 1. Of the upper and lower drums 1 and 2 constituting the cylinder C, the lower drum 2 is the stationary drum, and the upper drum 1 is the rotary drum rotatably arranged relative to the lower drum 1. The magnetic head 5 is mounted to the upper drum 1 and rotates along therewith and the sliding surface 6 runs on the magnetic tape 4. FIG. 2 in exaggerated scale illustrates the magnetic head 5 and its neighborhood, and FIG. 3 illustrates the upper drum 1 as turned upside down to reveal how the magnetic head 5 is mounted. In these figures, 1-5 represent the same parts as those of FIG. 1. A recessed portion 7 is provided in that parallel plane of the upper drum 1 which faces at the slit 3 with its one side opened in the peripheral surface thereof. The magnetic head 5 is fixedly secured within this recessed portion 7. 8 represents a magnetic gap of the magnetic head 5.
Next, the operation is described.
At first, for comparison, explanation is given to a case where the drum has no recessed portion 7 and no magnetic head 5 by reference to FIG. 4. Now suppose the magnetic tape 4 is trained round the cylinder C and the upper drum 1 rotates at a peripheral velocity V. When the upper drum 1 stands still, the magnetic tape 4, while remaining to be trained round the upper drum 1, is in contact with it. But, upon rotation of the upper drum 1, because air has a viscosity, the air layer on the surface of the upper drum 1 is drawn along to move. As a result, an air flow takes place between the upper drum 1 and the magnetic tape 4 in the same direction as that of rotation of the upper drum 1, causing the magnetic tape 4 to float up from the upper drum 1. And, the upper drum 1 and the magnetic tape 4 move relative to each other in spaced relation by some distance. This distance Hd is given by the following expression EQU Hd=0.643.multidot.Rd.multidot.(6.mu.V/T).sup.2/3 ( 1)
where RD is the radius of the upper drum 1, .mu. is the coefficient of viscosity of air, V is the relative speed between the upper drum 1 and the magnetic tape 4, T is the tension applied to the magnetic tape in the longitudinal direction.
If the recessed portion 7 exists in the upper drum 1 as in the prior art, however, the uniformity of the air flow between the upper drum 1 and the magnetic tape 4 is broken. Therefore, as shown in FIG. 5, the distance the magnetic tape 4 floats becomes different between the front and rear sides of the recessed portion 7. That is, now assuming that the magnetic head 5 is not in use, it is then ahead of the recessed portion 7 as viewed to the direction of rotation of the upper drum 1 that the distance Hd.sub.1 the magnetic tape 4 floats is equal to that Hd without the recessed portion 7, but behind it that the distance Hd.sub.2 becomes narrower than that Hd by about one figure. In accompaniment with this, the force that lifts the magnetic tape 4 upward is decreased at or near the position of the recessed portion 7. So, in some case, it happens that the magnetic tape 4 is looped below the cylindrical surface of the upper drum 1 into the interior of the recessed portion 7.
Such an influence of the presence of the recessed portion 7 on the lift-up amount of the magnetic tape 4 is produced regardless of whether or not the magnetic head 5 is installed. Therefore, in the conventional device shown by FIGS. 1 to 3, the decrease of the lift-up amount of the magnetic tape 4 by the presence of the recessed portion 7 is reflected to an increase of the apparent protruding amount of the magnetic head 5. This results in that the magnetic head 5 and the magnetic tape 4 are brought into contact with each other under a large pressure. Hence, the worn amount of the magnetic head 5 becomes large. Also, the damage of the magnetic tape 4 due to the running of the magnetic head 5 increases and, in the worst case, streaks are formed. Also, when the magnetic tape 4 sinks into the recessed portion 7, particularly at the position behind the magnetic head 5, the magnetic tape 4 will be scratched by the rear edge of the opening of the recessed portion 7. Further, the increase of the apparent length of protrusion of the magnetic head 5 with the decrease of the lifting-up distance of the magnetic tape 4 also causes the quantity of jitter to increase. In more detail, the magnetic head 5 enters under the magnetic tape 4 with an increased frictional resistance therebetween, and, therefore, vibrates the magnetic tape 4 in the lateral direction. This causes the increase of the quantity of jitter.
The extent of the above-described decrease of the lifting-up distance of the magnetic tape 4 due to the presence of the recessed portion 7 increases with decrease of the thickness of the tape. In particular, recently, the thickness of the magnetic tape is as far decreased as possible in order to increase the total recording time of the cassette for VTR. Therefore, the problem arising from the decrease of the above-described lifting-up distance due to the presence of the recessed portion 7 has been serious.
In the prior art, however, because the lifting-up distance differs across the width of the magnetic tape 4, there has arisen the following problem. That is, FIG. 6 illustrates the variation with width of the lifting-up distance as the magnetic tape 4 lies over both upper and lower drums 1 and 2 in almost evenly distributed relation. As is understandable from the figure, it is on the lower drum 2 that the lifting-up force is not exerted because it remains stationary. Therefore, the lower half of the width of the magnetic tape 4 does not float but contact on the peripheral surface of the lower drum 2. Meanwhile, the upper drum 1 produces the lifting-up force thereon, but the tape floating amount is not uniform because the circumstantial condition differs. Particularly as approaching to the peripheral edge of the inside parallel plane of the upper drum 1 or the slit 3, the floating amount decreases. Despite this, the magnetic head 5 is made placed adjacent the slit 3. Moreover, it is usual that it may project about several tens of microns outwardly of the peripheral surface of the upper drum 1. Because of this, the pressure under which the magnetic tape 4 and the magnetic head 5 contact with each other in a portion at or near the slit 3 necessarily becomes high. As a result, the magnetic head 5 is one-sidedly worn out. Also, from the same reason, the damage of the magnetic tape 4 is intensified. Also, the friction between the magnetic head 5 and the magnetic tape 4 and between the upper drum 1 and the magnetic tape 4 gives streaks to the magnetic tape 4 with a high possibility. The conventional rotary head device owing to such causes has been poor in durability and fidelity.
Also, the magnetic tape 4 makes an angle with the flat plane of the cylinder C when it is trained round. Therefore, that fraction of the width of the magnetic tape 4 which is on the upper drum 1 varies from the full width to zero depending on the angular position of the drum 1. And, because the floating amount of the magnetic tape 4 depends on the superimposed width of the magnetic tape 4 on the rotating upper drum 1, it takes a large value at the start (termination) of training, and a small value at the termination (start) of training. As a result, the spacing between the magnetic head 5 and the magnetic tape 4 is influenced to constantly change by the floating amount of the entirety of the magnetic tape 4, causing the envelope of the reproduced output signals to become uneven.